Electron multiplier



June 1942- F. J. G. VAN DEN B scH 2,285,848

ELECTRON MULTIPLIER Filed March 29, 1940 INVENTOR F.J. G. VAN DEN BoscH ATTYS.

Patented June 9, 1942 OFFICE ELECTRON MULTIPLIER In Great Britain Ma 11, 1939 2 Claims.

This invention relates to electron multipliers consisting of electron discharge devices in which one or more electrodes are employed for producing secondary electron emission in order to obtain the electron multiplication. Usually such electron multipliers comprise a primary cathode, one or more secondary cathodes and a collector enclosed in an envelope and so arranged that the electrons emitted by the primary cathode are caused to impinge on a secondary cathode and liberate other electrons which impinge on the next secondary cathode, when more than one, and so on in succession to the collector.

According to the present invention there is provided an electron multiplier comprising, for a secondary cathode, an auxiliary apertured electrode spaced from the secondary cathode in the direction of approach of electrons to this secondary cathode.

Further, according to the invention, there is provided an electron multiplier comprising a secondary cathode having apertures for the passage of electrons liberated by the secondary cathode to a succeeding electrode and an auxiliary apertured electrode spaced from the secondary cathode in the direction of approach of electrons to this secondary cathode.

In use of the multiplier according to the invention, the auxiliary electrode is operated at a potential which is the same, or substantially the same, as the operating potential applied to the associated secondary cathode. In this manner, there is produced in front of the secondary cathode a uniform field which very materially increases the multiplication factor obtained by the secondary cathode.

The auxiliary electrode is preferably uniformly spaced from the secondary cathode and it is convenient for the secondary cathode and the auxiliary electrode to be mounted one upon the other so as to be in direct electrical connection, and therefore operate at the same standing potential.

In cases where a secondary cathode having apertures for electrons is employed, this secondary cathode may be in the form of a wire grid or mesh or a perforated plate or of any other construction providing a surface on which approaching electrons may impinge, and an opening or openings through which the secondary electrons may pass to a succeeding electrode. The auxiliary electrode preferably consists of a wire grid r mesh or perforated plate with one or more apertures, constructed and arranged to offer little restriction to the passage of electrons to the secondary cathode. In one construction the secondary cathode consists of a perforated plate, and the auxiliary electrode consists of another perforated plate arranged with the perforations out of line with those of the secondary cathode so that electrons passing the auxiliary electrode are caused to impinge on the secondary cathode. In the case where the preceding electrode is also a secondary cathode consisting of a perforated plate, this plate is preferably arranged with the perforations of the plate in line with those of the succeeding auxiliary electrode so that the auxiliary electrode does not materially impede the passage of electrons.

A further feature of the invention consists in the provision of a control grid between the secondary cathode and the auxiliary electrode associated therewith. A control grid used in this position has the advantage that the electron emission at the secondary cathode is not affected by the control electrode.

Specific embodiments of the invention are illustrated, by way of examples, in the accompanying drawing, in which:

Figure 1 is a circuit diagram of a multiplier according to the invention,

Figure 2 is a perspective View of the multiplier employed in Figure 1,

Figure 3 is a diagram showing the provision of a control grid for a secondary cathode, and,

Figure 1 is a detail View of one form of secondary cathode and auxiliary electrode.

Referring to Figures 1 and 2 of the drawing, the electron multiplier comprises an envelope 10 containing a'thermiom'c cathode H and its heater l2. Associated with this cathode there is an input control grid 13 and an accelerator 14. The primary electrons from the cathode l I are directed towards the first one of a succession of secondary cathodes l5 which are in the form of perforated metal plates. There is a final secondary cathode Iii in the form of a disc and an output or collector electrode I! in the formof a grid interposed between the secondary cathode l6 andthe last of the secondary cathodes] 5.

In the construction shown in Figure 2 each of the secondary cathodes I 5 consists of a perforated metal plate, in which the perforations indicated at I8 are of diminishing cross-section and in which there is a peripheral flange l9 for electrostatically'focusing the electrons as they leave the secondary cathode,"as' described'in the specification of UnitedStatespatent application No. 276,883, filed June 1, 1939.

In front of each of the secondary cathodes Hi there is provided an auxiliary electrode which, in the construction shown in Figure 2, consists of a grid formed of very fine wire. Each of these auxiliary electrodes 20 is directly mounted in front of the associated secondary cathode so as to be in direct electrical connection with the cathode, but uniformly spaced in front of it.

In the amplifying circuit shown in Figure 1 an input transformer 2| has its secondary winding connected to the control grid [3 and cathode I l of the multiplier. A potential divider consisting of resistances 22, a further resistance 23 and a resistance 24 having a variable tapping is for connection at 25 and 26 respectively to the positive and negative poles of a high tension supply. The secondary cathodes l5 are connected respectively to the higher potential terminals of the resistances 22 and the accelerator I4 is connected to the higher potential terminal of the resistance 23 so that there is applied to these electrodes progressively increasing potentials with respect to the cathode which is connected to a tapping on the resistance 24. The negative terminal of this resistance is connected to the secondary winding of the transformer 2| so as to provide an operating bias on the control grid l3. An additional high tension supply is applied with its positive and negative poles at 2! and 25 respectively across a potential divider consisting of resistances 28 and 29. The junction of these resistances is connected to the secondary cathode l6 and the high potential end of the resistance 28 is connected through the primary winding of an output transformer 30 to the collector H.

In use, primary electrons from the cathode II are accelerated by the accelerator l4 towards the first of the secondary cathodes IS. The electrons pass through the auxiliar electrode in front of this secondary cathode to impinge upon the secondary cathode. Secondary electrons are emitted into the space between this secondary cathode and the auxiliary electrode. The field of the next following secondary cathode operating at higher potential penetrates through the perforations of the first secondary cathode to accelerate the secondary electrons towards the second of the secondary cathodes l5. The process is repeated for each of the secondary cathodes I5. The electron emission from the last of these secondary cathodes passes through the collector grid [1 to the final secondary cathode [6, the secondary emission from which is collected at the grid IT. The provision of the auxiliary electrode serves to screen the secondary electrons from the preceding emitting electrode, which has an operating potential negative to that of the secondary cathode from which the electrons came, and enables a considerable increase in multiplication to be obtained. Considering secondary electrons emitted by one of the secondary cathodes under impact of electrons from a preceding secondary cathode, these secondary electrons will be mainly or entirely at the front of the electrode emitting them and will have different velocities. They will be more or less influenced by the positive field of the succeeding secondary cathode for accelerating them towards the latter, but the effect of this field at the front of the cathode emitting the secondary electrons is, in the absence of an auxiliary electrode, reduced by the field of the preceding secondary cathode which is at a relatively negative potential. The auxiliary electrode serves to screen the space between this electrode and the associated secondary cathode, into which space the secondary electrons are emitted, from the infiuence of the preceding secondary cathode and permits the more active penetration of the accelerating field of the succeeding secondary cathode into this space.

In the modification shown in Figure 3 there is employed in the space between the secondary cathode I5 and the auxiliary electrode 2|] a control grid 3! which may conveniently be of wire grid or mesh form. Such a control grid 3| may have a modulating or control potential applied to it. For instance, it may be connected to a circuit tuned to required signal frequencies and furthermore it may be biassed or constructed for rectification of modulated carrier wave signals applied to the control grid I3 for amplification by the multiplier. These control grids 3! do not affect the secondary emission of the secondary cathodes with which they are associated because oi the provision of the auxiliary electrode 20.

Instead of employing a grid for the auxiliary electrode 2!] a perforated plate may be employed similar to that forming the secondary cathode. Such an arrangement is shown diagrammatically in Figure 4 in which the perforated plate forming the auxiliary electrode 20 has its perforations 32 staggered with respect to the perforations l8 of the secondary cathode with which it is associated, but in line with the perforations I8 of the preceding secondary cathode beneath it in Figure 4. Thus, the auxiliary electrode offers a minimum of obstruction to electrons approaching the associated secondary cathode and tends to direct such electrons so that they impinge on the secondary cathode.

In one construction which has been found satisfactory in practice in the case where the difference of potential of the order of 300 volts is employed between secondary cathodes, the secondary cathodes are spaced 6 to 8 mm. apart and the auxiliary electrode is positioned 3 or 4 mm. in front of the secondary cathode with which it is associated, that is to say, midway between successive secondary cathodes.

The invention is applicable also to electron multipliers having one or more secondary cathodes in which apertures are not provided and in which the secondary electrons are drawn away sideways from the surface of the cathode'instead of through the cathode. The invention is thus applicable to electron multipliers of the Zworykin type in which there is employed a plurality of secondary cathodes spaced apart in a single plane with deflectors for causing the electrons to follow a curved path between each of these secondary cathodes, and to electron multipliers of the Slepian type in which the secondary cathodes are arranged in two groups spaced apart, a cathode of one group being positioned opposite the space between adjacent cathodes of the other group, so that the electrons follow a zig-zag path between the cathodes of the two groups alternately.

I claim:

1. An electron multiplier comprising a primary cathode, a plurality of secondary cathodes each consisting of a perforated plate and a collector electrode, an auxiliary electrode positioned between successive secondary cathodes and also consisting of a perforated plate, the perforations in said auxiliary electrode'being in line with the 1 perforations of the preceding secondary cathode and staggered in relation to the perforations in the succeeding secondar cathode.

2. An electron multiplier comprising a primary cathode, a plurality of secondary cathodes each consisting of a perforated plate and a collector electrode, an auxiliary electrode positioned between successive secondary cathodes and also consisting of a perforated plate, the perforations in said auxiliary electrode being in line with the perforations of the preceding secondary cathode and staggered in relation to the perforations in the succeeding secondary cathode, and a control electrode interpositioned between the secondary cathode and the auxiliary electrode.

FRANCOIS JOSEPH GERARD VAN DEN BOSCH. 

